The invention relates generally to sensing technologies and, more particularly, to fiber-optic sensing modules for measurements of dynamic parameters in harsh industrial systems.
Combustion chambers such as are used in conjunction with gas turbines burn fuels in turbomachinery systems. The fuels may include natural gas or kerosene, for example, and are typically combined with air. As a result of the combustion process, such turbines emit an exhaust stream or gas containing a number of combustion products, including various forms of nitrogen oxide, collectively referred to as NOx, which is considered a pollutant. For a gas turbine, NOx emissions increase significantly as the combustion temperature rises. Operating a turbine in a so-called lean burn condition involves use of a lean mixture of fuel and air (that is, a relatively low fuel-to-air ratio) reduces the combustion temperature to a level that significantly reduces NOx emissions. Thus, sensing systems for monitoring temperature of the combustion chamber are useful in such conditions.
Power generation system combustion is closely monitored for efficiency control and optimization. During the combustion process, fuel and air are ignited and burned in a combustor, producing extremely hot gas at very high pressures. Dynamic pressure waves occur during the combustion process and have an acoustic frequency typically ranging from a few tens hertz to 30,000 hertz. If these dynamic pressure waves are not maintained at a sufficiently low vibration amplitudes, mechanical damage may occur in the combustion chamber. Further, a gas turbine lifetime decreases when the gas turbine's vibration amplitude is excessive.
Efforts have been made to monitor combustion dynamics, and one method includes employing a pressure transducer that includes a tube having one end projecting into the combustion chamber to be exposed to combustion pressure therein and another end with a piezoelectric crystal. The tube thus serves to reduce the amount of both pressure and temperature applied to the piezoelectric crystal to prolong the life of the pressure transducer. The pressure transducers are quite fragile, and frequently fail, causing difficulties and delays in testing of new turbines. Another method is to use a piezoelectric, piezoresistive, or capacitive based accelerometer or a velocity transducer to monitor a dynamic event to obtain data on vibration frequencies and amplitude. However, these types of sensors have limited usefulness in harsh environments because the piezoelectric, piezoresistive, and capacitive materials can not withstand very high temperatures.
A Fiber Bragg grating (FBG) is generally a periodic or quasiperiodic refractive index modulated structure that can be inscribed in a silicon dioxide-based photosensitive fiber to create a FBG sensor for measuring dynamic parameter, such as thermal ramping, dynamic pressure, vibration, and flow rate. Silicon dioxide has a high temperature melting property (typically higher than 1500 degrees Centigrade (2700 degrees Fahrenheit)). However, it is still challenging to use FBG sensors in harsh environments because conventional packaging for the fiber sensor protection and installation often cannot survive such harsh environments. Thus, there is a need in the art to provide a FBG based dynamic sensing module and packaging method to survive harsh environments and events.